This invention relates to engine start systems, and, more particularly, to a new and useful dual charge accumulator system for starting gas turbine engines.
An auxiliary power unit (APU) is a gas turbine engine that can supply rotary power to driving a generator and that supplies compressed air power for starting the main gas turbine engine(s) of a helicopter. In the present state of the art pertaining to rotary wing aircraft, helicopter engines are started with APUs or accumulators. The latter provide pressurized fluid instead of gas to perform work. Alternatively, the APU may in turn be started with an accumulator that supplies pressurized fluid to turn the compressor rotors of the APU in starting. Noncompressible hydraulic fluids are used in these accumulators.
Accumulators typically use a stored charge of compressed gas to propel their hydraulic fluid with sufficient force to perform useful work. Once the hydraulic fluid has been expelled and the compressed gas has been expanded, an accumulator must be recharged before it can be used again. Recharging is accomplished by gathering the hydraulic fluid under pressure and then compressing gas to a pressure equal to that of the gathered hydraulic fluid.
Although accumulators are capable of starting the main engines of a helicopter, it is usually more efficient and advantageous to use an APU for starting the main engines and an accumulator for starting the APU. To start the main engines of a helicopter, an accumulator would require approximately five gallons of hydraulic fluid. For the second start, or a failed start, recharging the accumulator by pumping the expelled fluid back into its reservoir under pressure and compressing a gas charge would require several hours of work using standard size manual or electric pumps. While heavier equipment might be used to reduce the time required for recharging on accumulator, the added weight against the time saved can cause the cost/benefit balance to tip in favor of standard equipment and longer recharging time. In either case, the situation is unfavorable.
Since an APU is much smaller than a main engine and requires for less power for start-up, the accumulator used to start the APU will require only about one gallon of fluid instead of the five typically needed for a main engine start-up. Smaller standard pumps for recharging the fluid and gas under pressure are therefore feasible.
For starting aircraft engines the capability of two start attempts provides a second starting opportunity in the event the first one fails. It is desirable that the attempts to made in rapid succession, to avoid wasting time between attempts while the aircraft may not be utilized and especially when one of the engines is idling and burning fuel to no useful purpose. One means of satisfying this requirement is to have two separate accumulators available. The alternative is to recharge a single accumulator and make a second or subsequent start-up attempt on the recharge. Neither alternative provides a clearly favorable cost/benefit balance. A requirement to carry, service and maintain a second accumulator would clearly be detrimental. Taking the time to recharge a typical accumulator of the prior art having only one start capability imposes obvious inefficiencies on the process of starting the engines of a helicopter. A favorable alternative to this problem is presented by this invention.
The present invention provides an accumulator system for turbine engine start-ups which is comparable is size and weight to an APU start system, and which provides instantaneous second start capability.